Contact member for a variable resistor



0d. 1970 P. F. GERWITZ 3,535,670

'- CT MEMBER FOR A VARIABL RE Filed May 2, 1968 v v TO. PAUL F. 'GERWITZ.

ATTORNEY.

l: \\\\\\\\\\\\\\\\\\\\\\mi\m W W United States Patent M 3,535,670 (CONTACT MEMBER FOR A VARIABLE RESISTOR Paul F. Gerwitz, West Covina, Calif., assignor to Spectrol Electronics Corporation, City of Industry, Calif, a corporation of Delaware Filed May 2, 1968, Ser. No. 726,009 Int. Cl. H01c 5/00 U.S. Cl. 338-143 4 Claims ABSTRACT OF THE DISCLOSURE A variable resistor including a helically formed resistance element and a contact member adapted to move along the resistance element under the influence of a slider, the slider being mounted so that upon reversal of direction of movement of the slider, mechanical backlash and translation of the slider and contact occurs, said contact having its resistance engaging end formed relative to the resistance element to minimize any variation in the electrical characteristic of the resistor in response to movement of the slider due to the backlash.

BACKGROUND OF THE INVENTION This inventit-on relates to variable resistors more commonly known as precision potentiometers. Resistors of this type employ a resistance element in combination with a contact member provided in turn with means for m0ving the contact member over the resistance element in order to vary the value of electrical resistance in a circuit including the resistor as a part thereof.

A popular type of precision potentiometers involves a resistance element arranged in the form of a helical coil mounted in the housing. The contact member is assembled Within a slider block in turn keyed to a rotary shaft also known as a rotor arranged centrally of the housing mounting the resistance element. The various parts are arranged so that rotational movement imparted to the shaft causes the slider block to follow a path determined by the helix of the coil defined by the resistance element. Under these circumstances, the contact member slides over the surface of the resistance element.

In the normal use of a potentiometer of the kind described, it is desirable that the parts be constructed so that the contact element may move across the resistance element both in a forward and a reverse direction depending upon the direction of rotary movement applied to the shaft. With the parts described, a reversal of movement of the slider block that occurs when the direction of rotation of the shaft is changed involves a certain amount of mechanical backlash. The axial position of the slider block relative to the individual turns on the resistance element is also different for opposite shaft rotation. In one type of potentiometer, the clearance between a protrusion on the slider block and the helical coil turns allows axial translation and causes two distinct areas of wear on the surface of the contact member. Only one area makes contact with the resistance element at a time depending on the direction of shaft rotation, the other area being in spaced relation to the resistance element.

The resistance element is, in fact, an assembly which consists of a mandrel member, usually a length of wire Patented Oct. 20, 1970 formed of copper material around which a relatively small diameter fine wire of electrically conductive material is helically wrapped. The fine wire has disposed about its surface a light coating of insulating material so that the turns of the helix formed by the fine wire may abut one another. The mandrel is likewise provided with a coating of insulating material. A suitable bonding ma terial is applied to the mandrel and fine wire to secure the parts against relative movement.

A portion of the periphery of the resistance element has the insulation removed so that when the assembly is subsequently coiled, a track formed about the inside of the helical coil is provided over which a contact element may ride.

The contact element is mounted in a slider block and contains proper leads so that wires connected to terminals disposed outside of the housing may form a part of a circuit in which the variable resistor or potentiometer is a component.

The mechanical backlash referred to above is objectionable because the electrical characteristics of the circuit are different depending on the direction of shaft rotation as a specific angular setting relative to the potentiometer housing is approached. In other words, as the shaft is rotated in the direction creating the backlash, the electrical resistance of the circuit may vary due to relative axial movement between the slider and the resistance element.

It is accordingly the chief object of this invention to provide a precision potentiometer in which the effects of mechanical backlash on the electrical characteristics on a circuit including the potentiometer as a component are either eliminated or reduced to a minimum.

It is an additional object of this invention to provide a potentiometer in which the contact element is so formed that translation or axial movement of the slider block occurring when the operating shaft of the potentiometer is rotated in opposite direction is used to overcome the effects of backlash.

SUMMARY OF THE INVENTION This invention involves a contact element construction for a precision potentiometer of the well-known multiturn type in which the end portion of the contact element that normally engages the resistance element is provided with a shape such that the movement imparted to the contact element as the operating shaft is rotated in opposite direction will substantially overcome the effects of .mechanical backlash, the area of engagement between the contact member and the particular. turn of wire on the resistance element is essentially the same and the electrical output of the potentiometer is substantially the same for either direction of shaft rotation within the region of axial movement or translation.

This is accomplished by imparting to the portion of the contact member in engagement with the resistance element a twist so that the axis of the portion in engagement with the resistance element is at an angle other than to the axis of the intermediate portion of the contact member which serves to connect the contact with the slider block in such a manner that a contact is continually urged into resilient engagement with the resistance element.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded view of a variable resistor or potentiometer equipped with the contact element serving as the subject of this invention;

FIG. 2 is an end view with the front cover removed of a potentiometer illustrating the manner in which the contact member engages the helically formed resistano" element;

FIGS. 3 and 4 are enlarged fragmentary views taken along lines III-III of FIG. 2 showing the relationship between the end of the contact member formed in accordance with this invention, the resistance element, and the portion of the slider block supporting the contact member during two different operating positions; and

FIGS. 5 and 6 are views similar to FIGS. 3 and 4 showing a conventional contact member for the purpose of comparison with the contact member of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawings for an illustration of a preferred embodiment of the invention, it will be noted that a precision potentiometer 10 of the kind under consideration is shown in exploded form in FIG. 1. The potentiometer includes a housing 12 of generally cylindrical configuration formed of a plastic material such as a phenolic composition that may be readily molded with a helical groove 14' formed about the interior surface 16 thereof. Terminal members 7 are shown extending through the housing 12 for connection with the resistance element to be later described and a movable contact member.

The resistance element comprises a mandrel 8 about which is helically wrapped a line wire 20. Before the wire 20 is assembled to the mandrel, the mandrel is provided with a coating of an insulating material. Thus, the mandrel forms an insulated support for the fine wire 20. It is also desirable to coat the fine wire 20 with a coating of insulation material. With the construction described, the turns of the fine wire may abut one another as they are helically assembled about the mandrel 18. After a predetermined length of resistance element has been formed, the parts are supplied with a bonding cement and the assembly thereafter wound into the form of a coil similar to the coil formed by the groove 4 located in the inner surface 16 of the housing 12.

Prior to assembly of the resistance element in the housing 12, a portion of the periphery of the assembly is cleaned free of insulating material so that the original material from which the wire is formed will then conduct current in a manner to be later described Thus a track along which a contact member may traverse for the purpose of supplying a variable electric output is provided.

The potentiometer includes, in addition to the housing 12, a rear cover 22 containing a bearing 24 arranged centrally thereof. The cover 2 is releasably secured to the end of housing 12 through fasteners 26 of a conventional type. Also included in the assembly illustrated in FIG. 1 is a rotary shaft 28 having a first end journaled in the bearings 24 and a second end extending beyond a journal 30 disposed in a front end cover 32. An appropriate knob, a calibrated dial or gear (not shown), may be secured to the end of the shaft projecting from the front cover for the purpose of imparting rotary motion to the shaft 28. Secured to the shaft 28 is a slider block support 34 in which a slider block 36 is mounted. The support 34 is formed of anodized aluminum and is assembled with the shaft 28 so as to rotate therewith. A longitudinal groove 38 is provided in support 34 and accommodates a lower portion of the slider block 36. The parts are so arranged that the slider block 36 may move relative to the support 34 within the confines of groove 38 as will be more apparent from a later description.

The slider block 36 is formed of a plastic material such as a thermosetting phenolic resin or diallyl phthalate and contains a transverse groove 37 defining opposed walls 40 and 42. Secured to the bottom of the groove is a contact member 50 which includes a base portion 52 to which a lead Wire 53 is connected, an intermediate portion 54 extending upwardly from the base portion and an end portion 56 having a concave configuration. The other end of lead wire 53 is connected to a slip ring 58 mounted on a plastic sleeve (not shown) secured to the shaft 28. The slip ring 58 cooperates with a brush element (not shown) which is mechanically and electrically connected to the rear terminal 17. Lead 53 is provided with sufficient play so that as the slider block 36 moves in groove 38 of the support 34, a continuing electrical connection between the contact element and the slip ring obtains.

Considering the contact member, this invention contemplates construction the end portion 56 of the contact in such a manner that the transverse axis of the end portion is disposed at an angle other than normal to the axis of the intermediate portion 54 of the contact member. In other words, the axis of the end portion is skewed relative to the longitudinal axis of the intermediate portion. The parts are assembled as shown in FIG. 2 so that when motion is imparted to shaft 28, slider block 36 will slide axially in groove 38 because the walls 40 and 42 loosely span a portion of a turn of the helix formed by the coiled resistance element As indicated in FIG. 1, grooves 14 are spaced so that the Walls 40 and 42 of slider block 36 may project in the spaces between the turns of the coil assembled in grooves 14, note FIGS. 3 and 4. When the shaft is rotated in a clockwise direction, wall 42 will engage against the insulated side of the resistance element with the end 56 of the contact member in engagement with the track formed on the resistance element. Once the slider block 36 has reached the end of its travel and it is desired to move the contact member in the opposite direction, counterclockwise motion is imparted to the shaft 28. Because of the clearance involved, a certain amount of movement is provided shaft 28 before end wall 40 engages the side of the turn of the resistance element against which it will engage as the slider block moves in the opposite direction. During this interval, the contact element moves because it is fixedly secured to the slider block. In moving the contact element, under the circumstances where the axis 60 of the end portion 56 is normal to the axis of the intermediate portion 54, note FIGS. 5 and 6 of conventional construction, a change in electrical resistance is involved, note distance x, FIG. 6. However, when the contact end portion 56 is shaped as shown in FIGS. 3 and 4, as well as FIG. 1, the effect of the changing position of the contact element is overcome and the electrical resistance stays substantially the same. It has been found that when the angle of skew is on the order of 6, improved performance of the resistor is obtained. It is felt that a skew angle between 3 and 12 will produce satisfactory results.

While I have described a preferred embodiment of the invention, it is to be understood the invention is not limited thereto since it may be otherwise embodied within the scope of the following claims.

I claim:

1. In a variable resistor, the combination including housing means forming an enclosure for the operating parts thereof, resistance means, comprising a helically wound member positioned within said housing means, shaft means disposed within said housing means, slider means carried by said shaft, contact means connected to said slider, said shaft means, slider means and resistance means being arranged so that rotary movement applied to the shaft means will cause the slider means to traverse a helical path with the contact means in engagement with the resistance means, said contact means including a substantially elongated intermediate portion and a resistance means engaging end portion, said end portion 5 having its transverse axis skewed relative to longitudinal axis of the intermediate portion.

2. The combination set forth in claim 1 wherein said slider is provided with radial projection means disposed between adjacent turns of the helically wound resistance member, said radial projection means being adapted to engage one side of the helically wound resistance member when rotational movement in first direction is imparted to the shaft means and to engage the oposite side of the helically wound resistance member as rotational movement in a reverse direction is applied to the shaft, said slider encountering axial movement relative to the resistance member upon reversal in the direction of rotational movement of said shaft 3. The combination set forth in claim 1 wherein the 15 angle of skew is between 3 and 12.

4. The combination set forth in claim 1 wherein the angle of skew is 6 References Cited UNITED STATES PATENTS 3,331,048 7/1967 Orozco 338-143 2,953,761 9/1960 Cother 338-143 2,361,010 10/1944 Cary et a1 338-202 X 3,314,036 4/1967 Kruse 338-20Q X LARAMIE E. ASKIM, Primary Examiner A. T. GRIMLEY, Assistant Examiner US. Cl. XJR. 33 8--202 

